docs: def helical_turbulent_fd_Mandal_Nigam(Re, Di, Dc, roughness=0):
      r'''Calculates Darcy friction factor for a fluid flowing inside a curved 
      pipe such as a helical coil under turbulent conditions, using the method of 
      Mandal and Nigam [1]_, also shown in [2]_.
          
      .. math::
          f_{curv} = f_{\text{str,turb}} [1 + 0.03{De}^{0.27}]
          
      Parameters
      ----------
      Re : float
          Reynolds number with `D=Di`, [-]
      Di : float
          Inner diameter of the coil, [m]
      Dc : float
          Diameter of the helix/coil measured from the center of the tube on one
          side to the center of the tube on the other side, [m]
      roughness : float, optional
          Roughness of pipe wall [m]        
  
      Returns
      -------
      fd : float
          Darcy friction factor for a curved pipe [-]
  
      Notes
      -----
      Claimed to be valid for all turbulent conditions with 
      :math:`2500 < De < 15000`. At very low curvatures, converges on the 
      straight pipe result.
  
      Examples
      --------
      >>> helical_turbulent_fd_Mandal_Nigam(1E4, 0.01, .2)
      0.03831658117115902
  
      References
      ----------
      .. [1] Mandal, Monisha Mridha, and K. D. P. Nigam. "Experimental Study on 
         Pressure Drop and Heat Transfer of Turbulent Flow in Tube in Tube 
         Helical Heat Exchanger." Industrial & Engineering Chemistry Research 48,
         no. 20 (October 21, 2009): 9318-24. doi:10.1021/ie9002393. 
      .. [2] El-Genk, Mohamed S., and Timothy M. Schriener. "A Review and 
         Correlations for Convection Heat Transfer and Pressure Losses in 
         Toroidal and Helically Coiled Tubes." Heat Transfer Engineering 0, no. 0
         (June 7, 2016): 1-28. doi:10.1080/01457632.2016.1194693.
      '''
      De = Dean(Re=Re, Di=Di, D=Dc)
      fd = friction_factor(Re=Re, eD=roughness/Di)
      return fd*(1. + 0.03*De**0.27)